A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing devices. The projection optical system causes interface and imaging of diffracted beams from the circuit pattern on the wafer.
The devices to be mounted on electronic apparatuses should be highly integrated to meet recent demands for miniaturization and low profile of electronic apparatuses, and finer circuit patterns to be transferred or higher resolution have been demanded increasingly. A shorter wavelength of a light source and a higher numerical aperture (“NA”) in a projection optical system are effective to the high resolution as well as a reduced aberration in the projection optical system.
An optical element, such as a lens and a mirror, when deforming in a projection optical system causes aberration because an optical path refracts before and after the deformation and light that is supposed to form an image at one point does not converge on one point. The aberration causes a positional offset and short-circuits a circuit pattern on a wafer. On the other hand, a wider pattern size to prevent short-circuiting is contradictory to a fine process.
Therefore, a projection optical system with small aberration should hold its optical element(s) without changing a shape and a position relative to the optical axis of the optical element in the projection optical system so as to maximize the original optical performance of the optical element. In particular, the projection lens tends to have a larger caliber and a larger lens capacity due to the recent high NA in the projection optical system, and easily deforms by its own weight. In addition, a recently extensively developed exposure apparatus that uses extreme ultraviolet (“EUV”) light (“EUV exposure apparatus”) requires the projection optical system to include a small number of reflective elements or mirrors for the EUV light having such a characteristically short wavelength as about 10 to 15 nm, and precision of shapes of these mirrors and positions of these mirrors relative to the optical axis are extremely strict.
The EUV exposure apparatus is used for exposure of a circuit pattern of 0.1 μm, and has very strict critical dimension accuracy. Therefore, the mirror's surface shape is permitted to have deformation of only about 0.1 nm or less. Therefore, a processed shape of a mirror should be precisely reproduced in the EUV exposure apparatus after the mirror is assembled into the exposure apparatus.
However, the mirror is made of such a soft parent material that even a force applied by a retainer that holds the mirror or holding power would deform the mirror by about 0.1 nm. In addition, the mirror does not reflect all the exposure light, but absorbs the exposure light of 30% or greater. The absorbed exposure light would become residual heat, thermally expand the mirror, and change a shape of the mirror and its position relative to the optical axis. In other words, it has been difficult to hold a mirror in a projection optical system and maintain desired optical performance without changing a shape of the mirror or its position relative to an optical axis.